Field of the Invention
The present invention relates to a control system for a hybrid vehicle and an electric vehicle powered by at least one motor, and especially to a control system for an electric vehicle having an electric oil pump that establishes hydraulic pressure driven by its own electric motor.
Discussion of the Related Art
JP-A-2006-161850 describes a hydraulic supply device that is applied to a hybrid vehicle having an engine and a motor as a prime mover, and configured to supply operating oil to a transmission while stopping the engine. According to the teachings of JP-A-2006-161850, the hydraulic supply device is comprised of a mechanical oil pump activated by the prime mover of the vehicle, an electric motor powered by a battery, an electric oil pump activated by the electric motor, a transmission mechanism to which the operating oil is supplied from those mechanical and electronic oil pumps and a pump driver that operates the electric motor. Specifically, when starting the electric motor by the pump driver, the electric motor is rotated at fixed speed for a predetermined period, and then the electric motor is rotated in a manner to output a fixed torque.
Thus, in the hydraulic supply device taught by JP-A-2006-161850, a speed of an electric motor is fixed to a predetermined speed when starting the electric motor of the electric oil pump. Therefore, even if the oil pump undesirably suck air, a resultant load change of the electric pump can be prevented.
Thus, according to the hydraulic supply device taught by JP-A-2006-161850, the speed of the electric motor is fixed to the predetermined speed when starting the electric motor of the electric oil pump. For this reason, even if the electric oil pump sucks air when starting the electric motor, the load change of the electric pump can be prevented so that the electric oil pump can be started smoothly. However, under a cold climate, a viscosity of the oil is increased to disturb the startup of the electric oil pump. Basically, according to the conventional art of the hydraulic system having a mechanical oil pump and an electric oil pump as taught by JP-A-2006-161850, an output power of the electric motor for driving the electric oil pump is considerably smaller than that of the prime mover for driving the mechanical oil pump. In the hydraulic system of this kind, if the electric motor is started under the condition that the viscosity of the oil is high at a low temperature, the electric oil pump may not be started smoothly due to shortage of electric power generated by the electric motor.
The present invention has been conceived noting the foregoing technical problems, and it is therefore an object of the present invention is to provide a control system for an electric vehicle configured to startup an electric oil pump smoothly by an electric motor other than a prime mover of the vehicle.